The subject matter disclosed herein relates to gamma cameras, and more particularly to methods for calculating linearity and uniformity maps for non-pixelated gamma cameras.
Non-pixelated gamma cameras typically include a single crystal scintillator formed from Sodium Iodide (NaI). During a gamma event, light emissions from the crystal are detected by an array of photomultiplier tubes (PMTs) to create a signal that is integrated by a computer to determine a location and total energy of the event. However, the signal is typically non-linear due to a curvature response of the PMT array and gaps between the PMTs. As such, the detected location of the event will vary from the actual location of the event, thereby resulting in a poor image created from the signal if not corrected.
To correct the location of the event, calibration maps are applied to the image data. Different types of maps may be used. An energy map translates the energy as measured to the actual energy. A linearity map translates the location as measured to the actual location. A uniformity map corrects imperfections or non-uniformities in the detector by applying a correction factor to smooth or even the image. Typically, these maps are created for each of a plurality of isotopes during initial calibration and stored in a computer for subsequent calibrations of the cameras and are specific to the particular cameras or systems.
However, as the camera ages and is subject to repairs including replacing the PMTs, the maps become less accurate. For example, inaccuracies in the maps may result in straight lines being detected as curved lines or cause deviations in isotope emission peaks. Accordingly, a person in the field must recalibrate the camera for each isotope that is to be detected by the camera. Such recalibration takes a significant amount of time. Moreover, it is often difficult to acquire data for each of the isotopes that may be detected by the camera.